1. Field of the Invention
This invention relates to an optical pickup device for condensing a laser light beam on a signal recording surface of an optical recording medium and a method for adjusting the optical pickup device.
2. Description of the Related Art
Recently, the tendency is towards higher recording density of a disc-shaped recording medium, such as an optical disc or a magneto-optical disc, as a recording device for a computer system or as a package media for music or video information. Among the methods for increasing the recording density of these recording media, there is a method of increasing the numerical aperture of an objective lens provided on an optical pickup device for reducing the spot size of the laser light beam radiated on the recording medium and hence the recording pit size for raising the recording density of the recording medium.
In general, the converged light spot size in an optical pickup device is given as .lambda./NA where .lambda. is the wavelength of the laser light radiated on the recording medium and NA is the numerical aperture of the objective lens. Thus, by increasing the numerical aperture of the objective lens, the laser light beam can be illuminated on the recording medium for reducing the converged spot size and hence the recording bit size for improving the recording density.
However, a limitation of approximately 0.6 is imposed on the numerical aperture of the objective lens because of manufacturing constraint on the non-spherical simplex lens used predominantly as an objective lens. On the other hand, if a larger numerical aperture is used in order that the tilt or warping of the recording medium or the wavefront aberration of the laser light beam caused by assembling precision of the optical pickup device will be comprised within an allowable range, the disc substrate needs to be reduced in thickness. In the case of a digital video disc (DVD), for example, the disc substrate thickness is set to approximately 0.6 mm.
There has also been proposed an optical system of an optical pickup device in which, by way of providing an objective lens unit having the numerical aperture of not less than 0.6, a solid immersion lens (SIL) as disclosed in U.S. Pat. No. 5,125,750 is arranged between the objective lens and the recording medium to provide a so-called two-lens sets for improving the numerical aperture to not less than 0.8.
However, with this objective lens unit, the SIL constituting one of the two lens sets needs to be maintained at an optimum distance from the recording medium (referred to herein as an air gap). If this air gap is changed significantly, there is produced spherical aberration on the surface of the recording medium thus deteriorating an output signal from the recording medium as the information signals while disabling recording and/or reproduction of the information signals for the recording medium.
On the other hand, in an optical disc recording/reproducing apparatus, it is necessary to maintain a constant value of the air gap and a constant value of the distance between the objective lens and the SIL for maintaining the constant values of the air gap and the distance between the objective lens and the SIL. For coping with these requirements, the objective lens and the SIL are position-adjusted for minimizing the spherical aberration at the time of assembling the two lens sets.
Moreover, in the optical pickup device, it is usually difficult to make direct visual inspection of the spherical aberration.